1. Field of the Invention
The present invention relates to a semiconductor integrated circuit having a diode for an AC-DC rectifier circuit, and in particular relates to a semiconductor device able to prevent currents in the substrate during rectifier circuit operation from generating to become noise sources for other integrated circuits in the substrate.
2. Description of the Related Art
The contact-free IC cards which have been coming into use in recent years do not incorporate a power supply, but receive a supply of electric power by means of electromagnetic radiation from an external source to operate internal circuitry. In such contact-free IC cards, an AC magnetic field at 13.56 MHz is generally supplied from outside, and an internal rectifier circuit rectifies this to obtain a DC power supply, serving as the internal power supply. The internal rectifier circuit normally comprises a bridge circuit with four diodes; in response to the magnetic field supplied to the antenna comprising a coil or other inductance element, the inductance element generates an induced current, and this induced current is supplied to the bridge circuit. Because the induced current is an AC current the polarity of which reverses according to the polarity reversals of the magnetic field, the rectifier circuit, comprising the bridge circuit, is necessary to rectify this AC current to obtain a DC current. A bridge rectifier circuit is described in Japanese Patent Laid-open No. 10-201239.
A diode comprised by a rectifier circuit in turn comprises a pn junction in a p-type well region, formed in a p-type substrate. This p-type well region is further formed within an n-type well region, with the aim of electrically isolating the well region from the p-type substrate. That is, the p-type well region within which the diode is formed has a triple well structure, being formed within an n-type well region formed in a p-type substrate. In order to form CMOS transistors, n-type well regions for p-channel transistors and p-type well regions for n-channel transistors are formed at the surface of a p-type substrate; in addition, another p-type well region is formed within an n-type well region to electrically isolate the p-type well region from the p-type substrate. Transistors and diodes which are not affected by the substrate potential are formed within such p-type well regions isolated from the substrate.
In general, a p-type substrate is connected to ground potential and is at a stable potential. In the case of a triple well structure, the n-type well region is set at a potential higher than ground, or at the same potential as the p-type substrate, so that the pn junction therebetween is not turned on.